The invention relates generally to a method and apparatus for automatically monitoring the temperature of one or more resistors in a current controller, and more particularly in a current controller for an electric propulsion system.
Large electric motor systems often employ resistance grids for controlling the current to the motors during acceleration and braking sequences of the motors. Control systems employing such resistance grids in connection with electric propulsion motors for transit vehicles are disclosed, for example, in U.S. Pat. Nos. 3,218,537; 4,458,185, and 4,697,124. In these propulsion systems, resistor grids in series and/or parallel with the propulsion motors are selectively varied for controlling the current to the motors during the acceleration and deceleration of the vehicles. Electric motors used in such propulsion systems are large and powerful, and require equally large amounts of current, which when resistance control is employed, passes through the acceleration and brake resistor grids. Due to the nature of these control systems, the resistor grids are only subjected to large currents only during a relatively short period during acceleration and braking sequences, on the order of 10 seconds. As may be appreciated, the large currents through the resistor grids generate substantial heat. The resistor grids are designed to handle such heat and traditionally are oversized to handle heat generated during a variety of normal operational circumstances. The design operating temperature of each grid is generally on the order of 450.degree. C. and the melting temperature of the grids in known systems is on the order of 1400.degree. C. Nevertheless, there are instances in abnormal operation and emergency operation, such as running up a large grade or in the towing of dead cars, when excessive heat can be generated which can cause failure in one or more of the resistor grids.
It would therefore be desirable to have a system for monitoring the temperature of the resistor grids in order to prevent overheating resulting in failure of the resistors.
It is theoretically possible to monitor the actual temperature of the resistor grids with the use of an instrument, such as a thermal couple associated with each resistor grid. However, the use of thermal couples is problematic due to the high voltages across the resistor grids. Furthermore, the use of thermal couples to monitor the temperature of the resistor grids which could include as many as 20 different sections, soon becomes excessively expensive.